Comparison of structural properties and capsaicin sensitivity among mechanosensitive colon afferent fibers in control (C57BL/6), TRPV1 knock-out, and ASIC3 knock-out mice
| Muscular | Mucosal | Muscular/mucosal | Serosal | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Strain | rf (mm2) | cv (m/s) | cap + | rf (mm2) | cv (m/s) | cap + | rf (mm2) | cv (m/s) | cap + | rf (mm2) | cv (m/s) | cap + |
| Control | 3.0 ± 0.3 | 0.70 ± 0.05 | 4.0 ± 0.3 | 0.74 ± 0.04 | 3.4 ± 0.3 | 0.76 ± 0.04 | 2.7 ± 0.8 | 0.69 ± 0.09 | ||||
| (n) | (10) | (9) | 6/10 | (11) | (11) | 3/6 | (12) | (10) | 8/11 | (10) | (9) | 2/9 |
| TRPV1 | 3.3 ± 0.4 | 0.73 ± 0.05 | 1.6* ± 0.4 | 0.60 ± 0.06 | 3.4 ± 0.3 | 0.70 ± 0.04 | 1.4 ± 0.4 | 0.58 ± 0.04 | ||||
| (n) | (14) | (8) | 0/8 | (9) | (8) | 0/8 | (21) | (11) | 0/9 | (12) | (8) | 0/6 |
| ASIC3 | 2.5 ± 0.4 | 0.37 ± 0.12 | 2.6* ± 0.4 | 0.71 ± 0.05 | 3.0 ± 0.4 | 0.75 ± 0.07 | 1.7 ± 0.4 | 0.55 ± 0.06 | ||||
| (n) | (11) | (3) | 3/8 | (15) | (8) | 4/10 | (14) | (5) | 1/8 | (12) | (4) | 2/8 |
All afferent fibers were determined to be C-fibers, with conduction velocities (cv) < 2 m/s. Receptive field area (rf) varied significantly based on fiber type and mouse strain (two-way ANOVA; afferent type, p = 0.007; strain, p = 0.018). Post hoc testing revealed that receptive fields of muscular/mucosal afferents were significantly larger than those of serosal afferents (p = 0.003) and that mucosal afferent receptive fields in TRPV1 knock-out and ASIC3 knock-out mice were significantly smaller than those in control mice (*p < 0.05 for both analyses). The capsaicin (cap) sensitivity of pelvic nerve colon afferent fibers was tested by application of capsaicin (3 μm, 2 min) to receptive fields at the end of the experiment to avoid potential desensitizing effects of the drug. A proportion of all four afferent fiber types in control and ASIC3 knock-out mice responded to capsaicin, although it did not activate any afferent fiber tested in TRPV1 knock-out mice. Data are expressed as mean ± SD.